1. Field of the Invention
The present invention relates to a magnetoresistance element and a manufacturing method therefor. In greater detail, the present invention relates to a magnetoresistance element having a large magnetoresistance (MR) rate of change, as well as to a manufacturing method for this element. The magnetoresistance element of the present invention is optimally applied to heads which reproduce magnetic signals written onto hard disks, floppy disks, magnetic tape, and the like.
2. Description of the Related Art
With respect to conventional magnetoresistance elements and manufacturing methods for such elements, the following technologies have all been reported by Inomata (Kouichiro Inomata: Ouyou Butsuri (applied physics), 63, 1198 (1194)).
FIGS. 5(b) and 6(b) are schematic diagrams showing the cross-sectional structure of conventional magnetoresistance elements. FIG. 5(b) shows an artificial lattice type structure (A) comprising a structure in which, on the surface of a substrate 501, there are a plurality of layers in which a nonmagnetic layer (spacer) 522 is sandwiched between ferromagnetic layers 521. Furthermore, FIG. 6(b) shows a spin valve type (B) having a structure in which, on the surface of a substrate 601, two ferromagnetic layers 621 are laminated with a nonmagnetic layer 622 therebetween, and an antiferromagnetic layer 623 is formed on the surface of the ferromagnetic layer which was provided last.
An example of the (A) artificial lattice type is a structure in which the nonmagnetic layer is formed from a Cr film, and the ferromagnetic layers are formed by Fe; that is to say, a Fe/Cr structure (FIG. 14, M. N. Baibich. et al.: Phys. Rev. Lett. 61, 2472 (1988)). FIG. 14 is a graph showing the change in resistance observed in three types of structures when an external magnetic field was altered. The vertical axis represents a standardization against the value observed in a magnetic field of 0. In the case of Fe/Cr, when the external magnetic field is at 0, the spin of the Fe layers is coupled in a mutually anti-parallel fashion and the resistance is high; it is noted that when a sufficiently large magnetic field (a saturation magnetic field, H.sub.s) is applied, the spin becomes mutually parallel and the resistance drops. The MR ratio at this time (the ratio of the change in resistance with respect to the resistance value in a saturation magnetic field) is 4.2 K and thus roughly 85%; even at room temperature, this is very large, at approximately 20%.
However, there is a problem in that although the Fe/Cr structure has a large MR ratio, H.sub.s is also large, at approximately 1.6.times.1.0.sup.6 A/m (20 kOe).
A structure in which the nonmagnetic layer comprises a Cu film and the ferromagnetic layer comprises a Co film, that is to say, a Co/Cu structure, has also been researched (D. H. Mosca. et al.: J. Magn. & Magn. Mater. 94, L1(1991), and S. S. P. Parkin. et al.: Phys. Rev. Lett. 66, 2152 (1991)). The MR ratio of the Co/Cu structure exceeds 50% at room temperature, and the H.sub.s is also smaller than that of the Fe/Cr.
Research was subsequently conducted on a number of artificial lattices; however, aside from the Fe/Cr system, the systems which have chiefly attracted attention are Co systems and Ni systems in which a noble metal is used as the spacer. Representative examples of these include, for example, Co/Cu, Co--Fe/Cu, Ni--Fe/Cu, Ni--Fe/Ag, and Ni--Fe--Co/Cu. Co systems exhibit a large MR ratio, while Ni systems exhibit a small saturation magnetic field. Fe/Cr systems have a spacer which is a transition metal, and have been studied in comparison with noble metal spacers in order to understand the mechanism thereof.
However, in considering magnetic head uses, it is desirable that H.sub.s be a few hundred Oe, and that a large MR ratio (at room temperature) be present; and there has been a problem in that in each of the systems described above, the confirmed MR ratio is insufficient.
One indication with respect to this problem is the report by the group of the present inventors that an MR ratio of approximately 6-7% was obtained in a Co/Cu system (Tsunoda, Takahashi, Miyazaki: Nihon Ouyou Jikigaku Kaishi Butsuri, 17, 826 (1993)). In this report, it was noted that it was possible to realize a MR ratio (at room temperature) of approximately 6.5% and a H.sub.s of approximately 0.2 kOe in a layered structure formed by the DC magnetron sputtering method, comprising a glass substrate and Cu (10 nm) [Cu(2.1 nm) Co(2.0 nm)] 20 Cu(10 nm). Furthermore, the MR ratio was found to be dependent on the Cu film thickness and the gas pressure. Additionally, waviness having a period of 10 nm to 50 nm was present in the surface of the laminated structure, and as the MR ratio decreased, the waviness of the structure increased, so that it is noted that in order to achieve a large MR ratio, it is a necessary condition that the structure be made flat.
Accordingly, the problem remains of determining film material and film formation conditions of each layer in order to make the structure flat.
Furthermore, Fullerton, et al. (E. E. Fullerton. et al.: Phys. Rev. Lett. 68, 859 (1992)), and Takanashi et al. (K. Takanashi et al.: J. Phys. Soc. Jpn. 61, 1169 (1992)) have reported mutually conflicting results with respect to turbulence at the interface between the nonmagnetic layer and the magnetic layer. Fullerton et al. have reported that the MR ratio increases when the turbulence at the interface is large. However, Takanashi et al. maintain that the MR ratio increases when the turbulence at the interface is small.
However, in the general report of Inomata it is noted that these two claims are not inconsistent and can be unified: "if scattering which is not dependent on spin is made as small as possible, and only that interface which contributes to spin dependent scattering is increased, MR will increase." Accordingly, in general terms, it has been discovered that the interface should be made flat.
Additionally, Inomata et al. have investigated the relationship between the degree of turbulence of the atoms at the interface and the MR ratio using NMR (nuclear magnetic resonance). In the film formation, the ion beam sputtering method was employed, and the acceleration voltage (VB) of the sputtered Ar ions was altered, and a Co/Cu superlattice was produced on a MgO (110) substrate. As a result, it was reported that in the test material exhibiting the greatest MR ratio, where VB=600 V, the interface mixing layer is a monatomic layer with respect to Co and Cu, respectively, and the component ratio of Co and Cu in the mixing layer is approximately 7 to 3 (K. Inomata et al.: J. Phys. Soc. Jpn. 62, 1450 (1993)). Accordingly, when the interface is microscopically apprehended, at the 1-2 atomic level, it is presumed that the presence of an appropriate mixing layer has the effect of increasing the MR ratio.
The (B) spin valve type represents a method for obtaining a large MR in a weak magnetic field. With respect to these types of methods, the fact that the methods shown below have been proposed by various institutions is reported in the general report of Inomata described above.
(1) A method in which the thickness of the nonmagnetic layer (spacer) is increased, the magnetic bonding between the magnetic layers is weakened, two types of magnetic layers having differing coercive forces (for example, Co and permalloy) are used, and only reversal of magnetization of the magnetic layer having the smaller coercive force is employed (T. Shinjo et al.: J. Phys. Soc. Jpn. 59, 3061 (1990)).
(2) A method in which anisotropy along one axis is provided at the surface of the film, which employs the sudden reversal of magnetization along the easy axis of magnetization (K. Inomata et al.: Appl. Phys. Lett. 61, 726 (1992)).
(3) A method in which an antiferromagnetic FeMn is employed, such as FeMn/NiFe/Cu/NiFe, the magnetization of the magnetic layers in contact with this is fixed, and only the reversal of magnetization of the other magnetic layers is employed (a non-bonding type spin valve film) (B. Dieny et al. Phys. Rev. B43, 1297 (1992)).
(4) A method which employs a multi-layered film of NiFeCo alloy, which is crystalline and magnetically anisotropic and is a weak magnetic material with little magnetic distortion, and Cu (J. Mouchot et al.: IEEE Trans. Mag. 29, 2732 (1993)).
(5) A method in which NiFe/Ag is subjected to heat treatment, and the NiFe layer is divided using the dispersion of Ag along the crystal granule interfaces of the NiFe (a non-continuous multi-layered film, or a granular multi-layered film) (T. L. Hylton et al.: Science 261, 1021 (1993)).
(1) through (4) above can be made to reflect the same understanding of the artificial lattice described above, that is to say, the ideas relating to the flatness of each layer and the turbulence at the interface. Only in the case of (5) above need a different mechanism be considered; it seems that here that a high MR ratio can not be considered.
As described above, research and development has progressed in each institution towards obtaining a high MR ratio. However, in the present state of affairs, in which higher and higher recording densities are achieved, the realization of a magnetoresistance element which is capable of conducting the reproduction of magnetic signals with high sensitivity, that is to say, the development of a magnetoresistance element having a MR ratio (at room temperature) which is higher than that conventionally obtained has been strongly desired. Furthermore, there has also been a desire for the development of a method which can stably produce such elements.
The present invention has as an object thereof to provide a magnetoresistance element which is capable of reproducing magnetic signals with high sensitivity, as well as a manufacturing method for such elements.